1. Field of the Invention
The invention is generally related to offshore drilling operations and more particularly to a riser support and guide mechanism for an offshore floating vessel.
2. General Background
In the drilling and production of hydrocarbons offshore, the development of deep water operations from floating vessels has included the use of tendons and risers under tension extending from the vessel to the sea floor. Such floating vessels have included tension buoyant towers, and spar structures in which the floating structures extend well below the surface of the water and are subjected to heave, pitch, and roll motions.
The lower ends of the tendons and risers are connected to the sea floor by means of additional pipes or risers embedded in and grouted to the sea floor. The upper ends of the tendons and risers pass through openings in the keel or bottom portion of the vessels and are supported vertically by tensioning means located near the water surface.
The openings in the keel serve to constrain the pipe forming the tendons or risers when the vessel is moved laterally with respect to the sea floor connection. Such lateral movement produces bending of the pipe at the constraint opening or rotation of the pipe about the contact of the pipe with the edges of the opening. Bending of the pipe which is normally under tension results in fatigue and wear at the constraint opening.
Riser pipe diameters can vary according to the functional requirements for the riser with typical designs varying from three to twenty-one inches. The opening in the keel guide support frame, for present designs, is sized to pass the connector used to tie the riser to the subsea wellhead. This connector diameter typically varies from twenty-seven to forty-eight inches, depending on the style of tieback connector used. Previous keel sleeves were designed to fill the twenty-nine to fifty inch hole provided in the spar keel riser frame. This resulted in a large diameter and thus very heavy and costly keel sleeve. This large diameter keel sleeve was generally too stiff to efficiently provide the bend limiting function that is desired. In addition, the length of the keel sleeve was required to be quite long (fifty to sixty feet) to ensure that the sleeve did not leave the keel guide as a result of relative motion between the floating structure and the riser.
Prior proposed means for controlling stress at such a point or area of rotation of the pipe have included tapered pipe wall sections of very large wall thickness. The thick tapered wall sections are usually machined from heavy forgings and are very expensive.
U.S. Pat. No. 5,683,205 discloses a stress relieving joint wherein a sleeve member is ensleeved over the pipe portion at the constraint opening and has an inner diameter greater than the outer diameter of the pipe portion. Means at opposite ends of the sleeve centralize the pipe within the sleeve such that the bending stresses at the constraint opening are relieved and distributed to the pipe at the ends of the sleeve member.
U.S. Pat. No. 5,873,677 discloses a stress relieving joint wherein a ball joint and socket assembly is removably attached to the keel at the constraint opening and a sleeve is attached substantially at its midpoint in the ball joint.
The known art does not address all aspects of riser support and guide mechanisms for floating offshore structures.